1. Field of the Invention
This invention relates to a rotary polygon mirror motor adapted to cause a laser beam to scan a given object at high speed and with high accuracy.
2. Description of the Related Art
In a laser printer or the like, a rotary polygon mirror motor adapted to rotate a rotary polygon mirror to cause a light beam to scan a given object, must be rotated at a high speed of several thousand revolutions per minute to several ten thousand revolution per minute with high accuracy. Especially, the flatness and the surface inclination of the rotary polygon mirror, and the vibration of the motor greatly affect the printing quality.
FIG. 3 is a vertical sectional view of a conventional rotary polygon mirror motor. In FIG. 3, reference numeral 1 designates a rotary sleeve; 2, a rotary polygon mirror which is secured to the rotary sleeve 1 through a cap 19 with screws 10. The mirrors of the rotary polygon mirror 2 (hereinafter referred to as "a rotary polygon mirror unit", when applicable) 2 are mirror-finished. The seat 2A of the rotary polygon mirror unit 2 which is mounted on the rotary sleeve 1 affects the surface inclination, and therefore its squareness with respect to the central axis of the rotary sleeve 1 is maintained considerably high in accuracy. Further in FIG. 3, reference numeral 3 designates a magnet yoke fixedly mounted on the rotary sleeve 1; and 4, rotary magnets. In general, in order to cause the magnetic flux of the rotary magnets to flow in the magnet yoke 3, the latter is made of a material such as iron which is high in magnetic permeability. The magnetic flux of the rotary magnet 4 is allowed to flow through stator cores 5 secured to a housing 9. Coils 6 are wound on the stator cores 5. The polarity of each of the rotary magnets 4 is detected with a Hall element 8 mounted on a base board 7. That is, the motor is an inner rotor type motor in which current is applied to particular coils to obtain torque.
The housing 9 has a stationary shaft 11 at the center, and has a particular groove pattern in the outer cylindrical surface, thus forming an air bearing with a gap of several microns (.mu.m) to several tens of microns (.mu.m) between the outer cylindrical surface thereof and the inner cylindrical surface of the rotary sleeve 1. In the direction of thrust of the motor, the rotary sleeve 1 is supported by the force of attraction of a thrust magnet 12 at one end of the stationary shaft 11 and another thrust magnet 13 at one end of the cap 19.
(1) In the above-described rotary polygon mirror motor, centrifugal load may act on it because of the high speed rotation, or heat may be generated therein by the windage loss thereof or by the loss of the motor. In this case, depending on the force of tightening screws which fix the rotary polygon mirror or depending on the number of the screws, the rotary polygon mirror may not be elongated uniformly in the direction of centrifugal load, or it may be elongated non-uniformly when thermally expanded. As a result, the rotary polygon mirror is greatly adversely affected in flatness or in surface inclination. Furthermore, when the rotary polygon mirror is cyclically cooled and heated, for instance the screws are loosened. As a result, the rotary polygon mirror is shifted, so that it is worsened in dynamic balance, thus being vibrated. That is, its rotation is unstable.
(2) Another adverse effect due to the generation of heat is as follows: When the thermal expansion coefficient of the magnet yoke attached to the rotary sleeve is smaller than that of the rotary sleeve, the thermal expansion of the rotary sleeve due to the generation of heat is suppressed by the magnet yoke. Because the rotary sleeve cannot deform outward, the inner surface of the rotary sleeve forming a bearing is deformed, so that it may be brought into contact with the stationary shaft. This fact limits the use of material for formation of the rotary sleeve.
(3) In forming the mirrors of the rotary polygon mirror unit, the mirrors are piled up and finished for the purpose of reduction of the manufacturing cost. The seat of the rotary sleeve through which the latter is set in the rotary polygon mirror, must be machined in such a manner that its squareness with respect to the rotary shaft is considerably high in accuracy; otherwise, during the rotation of the rotary polygon mirror, the latter is greatly adversely affected in surface inclination. In order to overcome this difficulty, in machining the seat, a method is employed in which, with the bearing and the motor section assembled, the assembly is rotated with its own thrust. However, this method is disadvantageous in that the bearing may be scratched.